Magnesium metal is most commonly obtained by the electrolysis of molten magnesium chloride (MgCl.sub.2). The concentration of the magnesium chloride fed to the cell should be as high as possible, e.g., 93% or more by weight. In addition oxychloride content, carbon content and heavy metal content such as iron, nickel, manganese, etc. should be as low as possible. For example, iron should be maintained below 600 ppm in the magnesium chloride fed to high efficiency sealed magnesium cells (designed for minimal sludge growth and removal) as otherwise a spongy iron deposit occurs which tends to create shorts and shorten cell life. The current efficiency of I.G. Farben type cells can be improved by about 2.5% through the use of purified feed, e.g., from about 78% to 80%. The I.G. Farben type low efficiency cells are designed to provide frequent sludging operation.
It is to be understood that molten magnesium chloride reacts strongly with the oxygen and moisture content of air with which it may come in contact. Such an occurrence causes release of hydrogen chloride fume to the workplace, a highly undesirable event, along with a concomitant increase in the oxygen content of the molten bath. Accordingly, contact of magnesium chloride electrolyte and air is to be avoided.
The treatment of magnesium chloride to remove oxygen by chlorination using iron catalyst results in residual iron as an impurity which may be present in the amount of 0.4% or more by weight. For efficient operation of electrolytic cells, particularly sealed cells, the iron content has to be reduced to a value not exceeding 600 ppm. The present invention is directed to a means and method for purifying molten magnesium chloride electrolyte.